Separation units, such as atmospheric distillation units, vacuum distillation units and product strippers, are major processing units in a petroleum refinery or petrochemical plant. Atmospheric and vacuum distillation units are used to separate crude oil into fractions according to boiling point for downstream processing units which require feedstocks that meet particular specifications. In the initial fractionation of crude oil, higher efficiencies and lower costs are achieved if the crude oil separation is accomplished in two steps: first, the total crude oil is fractionated at essentially atmospheric pressure, and second, a bottoms stream of high boiling hydrocarbons (the atmospheric resid) is fed from the atmospheric distillation unit to a second distillation unit operating at a pressure below atmospheric, referred to as a vacuum distillation tower. The reduced pressure in the vacuum tower allows the unit to separate the bottoms fraction from the atmospheric tower into fractions at lower temperature to avoid thermally-induced cracking of the feed.
The vacuum distillation unit typically separates the bottoms stream coming from the atmospheric unit into various gas oil streams which may be categorized according to the needs of the refiner as light vacuum gas oil, heavy vacuum gas oil or vacuum distillate. The undistillable residual or bottoms fraction leaves the vacuum distillation unit as a liquid bottoms stream. Additional information concerning the use of distillation in petroleum refining is to be found in Petroleum Refining Technology and Economics, Gary, J. H. and Handwerk, G. E., pp. 31-51, Marcel Dekker, Inc. (1975), ISBN 0-8247-7150-8 as well as Modern Petroleum Technology, 4th Ed., Hobson, Applied Science Publishers, 1973, ISBN 0-8533-4487-6 and numerous other works.
In atmospheric or vacuum distillation, lighter hydrocarbons are vaporized and separated from relatively heavier hydrocarbons. Although the heavier hydrocarbons may not vaporize, they may be carried into the lighter hydrocarbons because of entrainment. This is particularly the case within many commercial designs of vacuum towers in which the two phase feed stream to the tower is generally under turbulent conditions so that the separated resid droplets are easily entrained in the vapors that are being flashed off from the incoming feed stream. Entrainment is undesirable because first, the presence of high boiling or undistillable fractions may be undesired for their physical properties, e.g. viscosity, and second, because the entrained heavier hydrocarbons are typically contaminated with metal-containing compounds such as vanadium or nickel compounds, that can poison the catalysts used in downstream processing. While some metal contaminants enter the lighter fractions by vaporization, reduction of entrainment is a more effective method of reducing metals contamination as it is the heavier fractions in which these contaminants are concentrated. For this reason, the present invention may be applied to fractionation or distillation towers regardless of the operating pressure if the construction of the towers or their operating regimes have led to re-entrainment problems; it may be applied to atmospheric towers, vacuum towers and high pressure towers or any unit in which reduction of re-entrainment is desirable.
Distillation towers often use various tangential entry devices to impart centrifugal force to the two-phase feed entering the tower. The droplets not captured in the feed zone are entrained with ascending vapors from the flash zone immediately underneath the feed zone and pass to the wash zone above the feed zone. If stripper trays are positioned at the bottom of the flash zone, the swirling feed vortex will tend to entrain resid from the top stripper tray and increase the extent of liquid entrainment, depending in part, by the shear force of the feed vapors on the liquid/froth surface of the liquid pool on the tray.
Various steps have previously been used or proposed to reduce entrainment in vacuum distillation. Demisters or wire mesh pads may be installed at some point between the flash zone and a liquid draw-off point. Demister or wire mesh pads may not, however, be completely satisfactory because they may have a tendency to plug with heavy oil and other material, have a tendency to corrode, with holes resulting from the corrosion or simply be ineffective in reducing entrainment.
Methods other than demister pads have also met with only limited success in many applications. Conventional bubble-cap trays above the flash zone may cause the vapor to pass through liquid on the bubble-cap tray, thereby allowing vapor to re-entrain liquid droplets besides creating a pressure drop which may be excessive, particularly in a vacuum tower in which the total tower pressure drop (top to bottom) should be maintained as low as is feasible.
Chimney trays having a number of risers attached to a plate having holes, with a baffle attached to the top of each riser have also been used. Chimney trays are available that use two direction changes in the flow of the vapor/liquid to improve liquid/vapor separation have a lower pressure drop than bubble-caps but they may still not be completely effective in reducing entrainment.
U.S. Pat. Nos. 4,698,138 (Silvey) and 5,972,171 (Ross) describe de-entrainment trays for vacuum towers which are based upon risers to effect improved liquid/vapor separation. Another type of de-entrainment device which has been used in various applications has taken the form of a conical baffle with vertical sides which sits over a large diameter riser located at the top of the stripper section of the vacuum tower. While this device has been effective it is relatively large and may not be suitable for installation in existing units which do not have adequate vertical clearances.
A further problem may be encountered in vacuum towers used for petroleum distillation. The bottoms stream from the atmospheric tower is passed into the flash zone of the vacuum tower where a portion of the stream is vaporized and travels up into the rectification or wash section in the upper portion of the tower. The liquid (non-vaporized) portion of the feed falls onto the trays in the stripper zone in the lower portion of the tower and may be agitated into a froth by the ascending vapor stream from the lower stripper zone as well as by the turbulent incoming feed stream; the liquid elements of the froth may then be picked up and entrained by the ascending vapors and taken up with the lighter fractions into the upper portion of the tower.
A need therefore exists to devise an improved device to reduce the degree of re-entrainment of separated liquids into the vapor stream of a distillation tower or column, particularly in vacuum and atmospheric distillation columns between the flash zone and the stripper zone. The improved device should, at the same time, cause a minimal pressure drop appropriate to use in vacuum distillation units.